Giver the information about the "pinging" while tuning, I'd bet this is a classic issue of strings sticking in the nut. Do a couple of things to check this:
[1] tune the G and B strings to pitch using a tuner
[2] play a couple of bend licks on the G ONLY, bend towards the bottom E, don't touch the B
[3] check the open tuning of the G and B again
If the G AND B are BOTH out of tune, the most likely possibility is that the bridge is shifting, or the entire neck is moving somehow
If only the G has changed tuning, then the tension on the string has changed. Assuming the string is properly stretched to begin with, there are only two possible reasons
[1] the tuning head has let more string unwind from the shaft. In this case, the string will go flat. A tuning head cannot, IMO spontaneously wind MORE string onto the shaft to make a note go sharp. This "slippage" is generally not a defect in the head, although it is often identifed as such. It is often caused by one of two things. the first is Insufficent string wound onto the shaft itself ,or wound on without the required one loop above the hole and all the others below ,allowing the string to slip round the tuner shaft. If your strings were changed by a luthier, this shouldn't be a problem.
The second is the behevior of tuning "down" to the correct pitch, instead of "up" to the correct pitch. If you are tuning and go too high in pitch, always go back down below the correct pitch and then back up. That way, the worm gear on the head is resting firmly on the gear at the bottom of the shaft.
[2] the most likey reason is that the string is sticking in the nut. if the nut is too narrrow, or 'V' shaped at the base of the slot, no amount of lubrication will stop it from grabbing the string. IMO a good nut slot has a nice curved bottom and is polished until it's shiny. This can be done with abrasive cord, or string soaked in a mild abrasive. A good luthier should know how to do this.